Much of the research and development effort in the field of automotive finishes is currently directed to the search for coating compositions and methods of applying such compositions which not only will eliminate, or nearly eliminate, the release of organic solvents during heat curing but also will produce at a commercially feasible cost, coatings at least comparable in appearance and durability to conventional coatings.
A substantial amount of current research effort is likewise directed to the development of clear coat/color coat automotive finishes. It has been found that an excellent appearance, with depth of color and with metallic glamour, can be obtained by applying a transparent coat over a pigmented coat. Unfortunately, the durability of these transparent clear coats has left much to be desired. Often, checking, cracking, and flaking occur after relatively short periods of exposure to weathering, necessitating costly refinishing.
One solution to the solvent emission problem has been the replacement of liquid coating materials with coating materials in the form of dry, particulate solids, commonly called "powder" coatings. These compositions contain very low concentrations of volatile solvents, i.e., of the order of 2 percent or slightly higher, substantially less than any other paint system.
From an environmental standpoint, powder coatings have much to recommend them. Inherent in their use, however, are certain problems of production and application which have retarded the extent of their adoption. One problem occurs when powder coatings are used in conjunction with particulate metal particles, e.g., aluminum flakes. Automobiles coated with a so-called "metallic" finish, i.e., a topcoat of enamel or lacquer in which aluminum flakes as well as conventional pigments have been dispersed, have found wide acceptance in the marketplace. For the most part, the problems incidental to employing aluminum flakes in conventional liquid paints have been solved through years of experimentation and use. The problems associated with the use of aluminum flakes in dry powder are far from complex, particularly where some type of pulverizing step is involved in the paint manufacturing process or where electrostatic spray techniques are used to apply the paint to a substrate. Also, although increased use of powder coatings and improved manufacturing methods will undoubtedly result in a reduction of the present cost of quality powder coatings, the cost of producing such coatings in all of the colors demanded in the marketplace may continue to be prohibitive.
In view of the problems associated with colored powder coatings, particularly those containing metallic flakes, one approach has been to utilize a clear coat/color coat system wherein the transparent clear coat is composed of a nonpigmented powder coating while the color coat, often metallic, is composed of a conventional liquid paint. Such a system possesses a number of advantages. Use of a powder coating for the clear coat reduces the solvent emission level considerably, and, if desired, a water-based or high-solids coating material can be used for the color coat in order to further reduce the total emission level. Thus, an environmentally-acceptable finish can be achieved without sacrificing appearance or metallic glamour. Too, the production of nonpigmented powder coatings is markedly less complex and less expensive than the production of pigmented powder coatings.
Clear coat/color coat systems consisting of a powder clear coat over a conventional liquid color coat are known in the art, as shown by Camelon et al., U.S. Pat. No. 3,953,644, issued Apr. 27, 1976. However, such systems are not reinforced against the destructive effects of outdoor weathering and are thus susceptible to the checking, cracking, and flaking which commonly beset clear costs.
Conventional ultraviolet light screeners have sometimes been added to liquid clear coats in an attempt to retard the degradation caused by weathering, as shown by La Berge, U.S. Pat. No. 3,407,156, issued Oct. 22, 1968. Many of these conventional screeners are unsuitable for use in a powder coating because they lack special requirements, e.g., the capability of being ground to small particle size, of melting readily at the standard bake temperature for powder finishes, or of having sufficient permanence to remain in the coating film during baking and subsequent outdoor exposure. In addition, the use of conventional ultraviolet light screeners is in some polymer systems less than satisfactory: the durability of certain clear coats so reinforced will be increased for a short period of time, but not to the extent required for a practical automotive finish.
Thus, there is a continued need for nonpigmented powder coating compositions which will provide clear coats characterized by both excellent appearance and the capability to adequately withstand long periods of outdoor weathering.